A number of microporous crystalline aluminium phosphates exhibiting molecular sieve properties have been described in European Patent 43,562. These products correspond to the general formula: EQU m R:Al.sub.2 O.sub.3 :(1.+-.0.2)P.sub.2 O.sub.5 :n H.sub.2 O
where R denotes an organic structuring agent, amine or quaternary ammonium salt, occluded in the channels of the structure, m and n denote the number of moles of R and of H.sub.2 O respectively per mole of Al.sub.2 O.sub.3. The Al/P molar ratio of these materials is practically equal to 1. The positive charge PO.sub.2.sup.+ associated with the phosphorus of the tetrahedra is balanced by AlO.sub.2 associated with the presence of aluminium in the tetrahedra. As a result of this, these products do not exhibit ion exchange properties.
On the other hand, once calcined to remove the organic species, these materials exhibit a relatively marked hydrophilic character and adsorb various other molecules. On the other hand, the catalytic properties remain very limited, at least insofar as acid-base catalysis is concerned.
The incorporation of silicon into these microporous aluminium phosphates is described in Patents EP-B-103,117, EP-A-146,384 to 146,389, EP-A-147,991, EP-A-154,122, EP-A-185,525 and GB-B-2,155,916.
The silicoaluminophosphates thus obtained are molecular sieves and have ion exchange properties. They can be converted, generally by simple calcination, into solids exhibiting catalytic properties.
The synthesis of silicoaluminophosphates is performed by a hydrothermal route, by heating to 100.degree.-200.degree. C. for 5 hours to 20 days, optionally with agitation, a reaction mixture comprising an organic structuring agent R and sources of aluminium, of silicon and of phosphorus. The initial pH of the medium is 3-8, the final pH is 8-10. The solid obtained corresponds to the formula: EQU R.sub.m ':Si.sub.x Al.sub.y P.sub.z O.sub.2 :n'H.sub.2 O
where R is the organic structuring agent, m' corresponds to the number of moles of structuring agent, n' to the number of moles of water per mole of Si.sub.x Al.sub.y P.sub.z O.sub.2, and x, y and z correspond to the molar fractions of silicon, aluminium and phosphorus.
The maximum value of m' depends on the nature of R, more precisely on its size and its charge, as well as on the pore volume of the microporous solid. The value of m' is generally between 0 and 0.3 and n' varies between 0 and 0.3.
The molar fractions x, y and z are such that x+y+z=1. The value of x generally lies between &gt;0 and 0.3, y between 0.2 and 0.6 and z between 0.2 and 0.6.
Compensating cations, generally alkali metal or alkaline-earth metal cations may be present in the structure. These cations originate from the synthesis medium or from an ion exchange after calcination. They take part with the organic cations R.sup.+ or with the protons in compensating the negative charge of the structure resulting from the incorporation of silicon in the tetrahedra.
The silicoaluminophosphates obtained by this process exhibit crystallization defects which are sometimes at the source of their lack of heat stability.
We have now found a process of synthesis which makes it possible to improve the crystallinity and the purity of silicoaluminophosphates, and this appreciably improves the heat stability of the products obtained.